Chemical-mechanical planarization ("CMP") processes remove materials from the surface layer of a wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer presses against a polishing pad in the presence of a slurry under controlled chemical, pressure, velocity, and temperature conditions. The slurry solution has abrasive particles that abrade the surface of the wafer, and chemicals that oxidize and/or etch the surface of the wafer. Thus, when relative motion is imparted between the wafer and the pad, material is removed from the surface of the wafer by the abrasive particles (mechanical removal) and by the chemicals in the slurry (chemical removal).
CMP processes must consistently and accurately produce a uniform, planar surface on the wafer because it is important to accurately focus optical or electromagnetic circuit patterns on the surface of the wafer. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the photo-pattern to within a tolerance of approximately 0.5 .mu.m. Focusing the photo-patterns to such small tolerances, however, is very difficult when the distance between the emission source and the surface of the wafer varies because the surface of the wafer is not uniformly planar. In fact, several devices may be defective on a wafer with a non-uniform surface. Thus, CMP processes must create a highly uniform, planar surface.
In the competitive semiconductor industry, it is also desirable to maximize the throughput of the finished wafers and minimize the number of defective or impaired devices on each wafer. The throughput of CMP processes is a function of several factors, one of which is the rate at which the thickness of the wafer decreases as it is being planarized (the "polishing rate") without sacrificing the uniformity of the planarity of the surface of the wafer. Accordingly, it is desirable to maximize the polishing rate within controlled limits.
One problem with current CMP processes is that the polishing rate varies over a large number of wafers because certain structural features on the planarizing surface of the pad vary over the life of a pad. One such structural feature is the non-uniformity of the distribution of filler material throughout the pad. Prior art polishing pads typically are made from a mixture of a continuous phase polymer material, such as polyurethane, and a filler material, such as hollow spheres. Shown in FIG. 1 is a prior art polishing pad 10 having spheres 12 embedded in a polymeric matrix material 14. As can be seen, the spheres 12 have agglomerated into sphere clusters 16 before the matrix material 14 fully cured, resulting in a non-uniform distribution of the spheres 12 in the matrix material 14. Consequently, regions on the planarizing surface 18 of the polishing pad 10 at the sphere clusters 16 have a high polishing rate, while regions that lack spheres have a conversely low polishing rate. In addition, when using such a polishing pad 10 in a CMP process, the planarizing surface 18 is periodically removed to expose a fresh planarizing surface. The density of sphere clusters 16 vary throughout the thickness of the polishing pad 10, thereby causing the polishing pad 10 to exhibit different polishing characteristics as layers of planarizing surfaces are removed. Although many efforts have been made to provide uniform porosity throughout the continuous phase material, many pads still have a non-uniform porosity on their planarizing surface. Moreover, the non-uniform areas of the pad are not visibly distinguishable from other areas on the pad, making it difficult to detect and discard unacceptable pads.